Enzymatic resolution of alpha-tertiary carboxylic acid esters

Abstract

 A method for the resolution of α-tertiary carboxylic acid esters by partial hydrolysis is disclosed. The partial hydrolysis is carried out by contacting the esters with an aqueous solution in the presence of an enzyme, and the hydrolysis product is separated from unreacted starting material to obtain the hydrolysis product or the unreacted starting material in enantiomerically enriched form. A novel, purified enzyme which is useful in this method, and a nucleotide sequence encoding this enzyme, are also described.

Description

[0001] This invention relates to the enzymatic resolution of α-tertiary carboxylic acid esters; more particularly, it relates to a method for preparing optically active acids and their corresponding esters by partial enzymatic hydrolysis of α-tertiary carboxylic acid esters using an enzyme derived from Candida lipolytica. The method of this invention is useful in preparing compounds which have utility as starting materials and intermediates for the synthesis of pharmaceuticals, and agricultural and veterinary products.

[0002] Review articles on enzymatic synthesis, such as D.H.G. Crout and M. Christen, Modern Synthetic Methods, 1989, vol. 5, R. Scheffold (Ed.), Springer-Verlag, and J.B. Jones, Tetrahedron, 1986, 42, 3351, describe many examples of the enzymatic resolution of esters in which the α-carbon possesses one hydrogen. However, relatively little has been reported for the enzymatic resolution of carboxylic acids or esters in which the α-carbon is fully substituted with moieties other than hydrogen.

[0003] Chenault et al., J. Am. Chem. Soc., l989, 111, 6354 describe the resolution of eight 2-amino-2-methyl carboxylic acids by partial hydrolysis of the amide moieties of racemic 2-N-acylamino-2-methyl carboxylic acids with acylase I from porcine kidney and from the fungus Aspercillus species. The enzymes display (S)-stereoselectivity, and three of the L-2-methylamino acids were prepared in greater than 90% enantiomeric excess. Results of the resolution of the N-acylmethyldopa derivative were not disclosed. The reported enzymatic resolutions require as much as twice the weight of enzyme to substrate and about eleven days to achieve 50% hydrolysis, and are therefore limited in their applications and in their usefulness in commercial processes.

[0004] Sugai et al., J. Org. Chem., 1990, 55, 4643,describe the resolution of the racemic methyl ester of 2-benzyloxy-2-methyl-4-pentenoic acid (an α-oxygen substituted ester) to yield the optically pure (S)-acid which was subsequently used in the synthesis of (1S, 5R)-(-)-frontalin, a constituent of the aggregation pheromone of the female southern pine bark beetle. The enantiomerically pure (S)-acid was obtained by enzymatically hydrolising the racemic ester with Candida cvlindracea lipase, isolating and then re-esterifying the enantiomerically enriched (S)-acid, and resubmitting the optically enriched (S)-ester to Candida cvlindracea lipase hydrolysis. This sequence for preparing (S)-2-benzyloxy-2-methyl-4-pentenoic acid requires two enzymatic resolution steps which result in longer overall reaction times and lower chemical yields than a process which employs only one enzymatic resolution step.

[0005] Enzymatic resolution of α-tertiary 2-hydrazino-2-methyl carboxylic acid esters has not previously been reported in the literature. Such compounds are useful in the production of amino acid decarboxylase inhibitors, and in particular carbidopa, which is used in the treatment of Parkinson's disease. The current methods of production of compounds such as carbidopa employ chemical resolution of 2-hydrazino (U.S. patent 3,895,052) or 2-amino (U.S. patent 3,405,159) precursors. Such methods require expensive chiral auxiliaries or costly and complicated equipment. See also U.S. Patent 3,830,827, which describes a chemical method for preparing carbidopa.

[0006] It has now been discovered that the enzyme obtained from Candida lipolytica is effective in resolving a variety of α-tertiary carboxylic acid esters of general formula


wherein R₁ and R₂ independently represent alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, arylalkyl or substituted arylalkyl, or R₁ and R₂ together are linked to form a cyclic structure incorporating the asymmetric α-carbon atom; R₃ represents an alkyl moiety having from 1 to 8 carbon atoms, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, arylalkyl or substituted arylalkyl; X represents alkyl, substituted alkyl, halogen, amino, alkylamino, arylalkylamino, acylamino, hydrazino, alkylhydrazino, arylalkylhydrazino, arylhydrazino, acylhydrazino, hydroxyl, alkoxy, arylalkoxy, aryloxy, mercapto, alkylthio, arylalkylthio or arylthio; and Y represents oxygen or sulfur; provided that R₁, R₂ and X are different from each other. The enzyme from Candida lipolytica has now been found to hydrolyze with high selectivity one isomer of a mixture of enantiomers of α-tertiary carboxylic acid esters. The resolution of these esters is accomplished by the partial hydrolysis of a solution or suspension of the ester in a predominantly aqueous mixture in the presence of the enzyme derived from Candida lipolytica, and separating the product acid from the starting ester. Separation is advantageously accomplished by methods well known to skilled practitioners of this art, such as by extraction, chromatography and selective precipitation. The selectivity of the present method permits the hydrolysis of predominantly one enantiomer, and preferably only one enantiomer.

[0007] In one aspect, the method of this invention can be applied to the enzymatic resolution of 2-hydrazino and 2- (β-N-acylhydrazino) esters, and in particular, alkyl 2-hydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl) propionates and alkyl 2-(β-N-acylhydrazino)-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionates, the ester precursors of carbidopa. For the optical resolution of these esters, the method is carried out in aqueous ester is hydrolyzed. The (S)-acid and (R)-ester can be conveniently separated, and the enantiomeric excess of each is at least about 99%.

[0008] In another aspect, the method of this invention can also be applied to the resolution of 2-amino-2-methyl carboxylic acid esters, such as alkyl 2-amino-2-methyl-3-(3,4-dihydroxyphenyl)propionates, which are precursors to the antihypertensive agent methyldopa. Enzymatic hydrolysis of the ester function of methyldopa esters affords (S)-methyldopa and the corresponding (R)-alkyl esters.

[0009] Other compounds which can be enzymatically resolved using the method of this invention include α-tertiary 2-hydroxy carboxylic acids such as racemic 2-benzyloxy-2-methyl-4-pentenoic acid.

[0010] A further aspect of this invention includes a novel enzyme derived from Candida lipolytica and having the N-terminal sequence as described herein, as well as nucleotide sequences encoded said enzyme.

[0011] The method of this invention is convenient, requires no costly or complicated equipment, and can be readily adopted for large-scale production of enantiomerically enriched α-tertiary carboxylic acids and their corresponding esters. The use of Candida lipolytica enzyme also has the advantages of greater hydrolytic activity and enantioselectivity than Candida cylindracea lipase with respect to the synthesis of 2-hydroxy-2-methyl or 2-alkoxy-2-methyl carboxylic acids, resulting in simplified and higher-yielding processes.

[0012] The compounds of this invention are useful as intermediates in the synthesis of pharmaceuticals,

[0013] The chiral acids and their corresponding esters of this invention can be generally obtained as follows. A racemic carboxylic acid ester of the general formula


wherein R₁ and R₂ independently represent alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, arylalkyl or substituted arylalkyl, or R₁ and R₂ together are linked to form a cyclic structure incorporating the asymmetric α-carbon atom; R₃ represents an alkyl moiety having from 1 to 8 carbons atoms, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, arylalkyl or substituted arylalkyl; X represents alkyl, substituted alkyl, halogen, amino, alkylamino, arylalkylamino, acylamino, hydrazino, alkylhydrazino, arylalkylhydrazino, arylhydrazino, acylhydrazino, hydroxyl, alkoxy, arylalkoxy, aryloxy, mercapto, alkylthio, arylalkylthio or arylthio; and Y represents oxygen or sulfur; provided that R₁, R₂ and X are different from each other, is dissolved or suspended in water or an aqueous buffer so that the final concentration of the substrate ranges from 0.001 to 6.0 moles per liter of reaction volume. The molarity of the buffer may range from 0.001 to 0.5 M, but to facilitate the isolation of the product acid, water or 0.01 to 0.05 M buffer is preferred. The concentration of the buffer conducting the enzymatic hydrolysis or product isolation. The pH of the resulting solution or slurry is adjusted to between 4 and 8.5, but a pH range of 7 to 8 is preferred. The aqueous medium can also include up to about 75%, but preferably less than 15%, by volume of a miscible organic solvent, including but not limited to methanol, ethanol, 1-propanol, 2-Propanol, acetone, 2-butanone, acetonitrile, dimethylformamide or dimethylsulfoxide. The use of a miscible orgranic solvent may serve to improve the solubility of certain substrates.

[0014] The enzyme is added to the substrate mixture to initiate the reaction and the resulting mixture is agitated. The optimum amount of enzyme used varies with the protein content of the enzyme preparation, the substrate and the reaction temperature, and a range of from 1 to 200 mg of protein per gram of substrate is preferred if crude enzyme is used. The optimum reaction temperature at which the hydrolysis is conducted may vary, and can range from 0°C to 55°C, but 15°C to 37°C is preferred. The atmosphere above the reaction may be air, or an inert gas, such as nitrogen. If the reaction proceeds for more than two days or at elevated temperatures, an inert atmosphere is preferred.

[0015] The enzyme used in this invention is commercially available and is derived from the yeast Candida lipolytica which is also known as Yarrowia lipolytica. A particularly preferred Candida lipolytica enzyme for the purposes oi this invention is that which is commercially available from Fluka Chemie AG, sold under the name "Lipase from Candida lipolytica". The commercial enzyme is actually believed to be a mixture of at least six electrophoresis (SDS-PAGE), various salts and buffer components and the like, as well as water insoluble material.

[0016] The crude enzyme that is obtained from commercial suppliers may be used without modification in the resolution of the Present substrates. Equally effective in resolving these substrates is the enzyme that has been either partially or fully purified from commercially available crude lipase or from the fermentation broth of an appropriate Candida lipolytica strain. Partial purification of the ester hydrolase may be accomplished by chromatography on a variety of resins including, but not limited to, ion exchange, hydrophobic, dye-ligand and hydroxylapatite.

[0017] A two-step process may be employed to purify the ester hydrolase to greater than 90% homogeneity from commercially available lipase preparations. Following an extraction of the crude lipase into a suitable buffer and removal of insoluble components, a protein fraction containing the ester hydrolase activity may be precipitated with polyethylene glycol. The extraction may be carried out using any standard biological buffer known to those skilled in the art, such as Hepes, phosphate or Tris, in the pH range of 6.0-9.0, with the preferred buffer solution being 10 mM phosphate, pH 6.5, containing 1.0 mM EDTA. Insolubles may be removed from the extraction mixture by methods standard in the art including either filtration of centrifugation. Purification of the ester hydrolase from the isolated protein fraction may be accomplished by column chromatography using a variety of resins with the mM phosphate, pH 6.5, containing 1.0 mM EDTA. After washing the resin with buffer to remove contaminating proteins, the ester hydrolase may be eluted from the dye resin with a high ionic strength buffer. The preferred buffer for elution of the ester hydrolase is 10 mM sodium phosphate, pH 6.5, containing 1.0 mM EDTA and 1.2 M KCl.

[0018] The ester hydrolase may also be purified to >90% homogeneity from the fermentation broth of an appropriate Candida lipolytica strain. Following the removal of cells from the fermentation broth, a protein fraction containing the ester hydrolase may be isolated by precipitation with polyethylene glycol or by chromatography on a suitable resin known to those skilled in the art. One chromatography method of isolation is hydrophobic chromatography on phenyl sepharose resin (Pharmacia, Piscataway, NJ). The ionic strength of the fermentation broth may be increased with a salt such as 1.8 M ammonium sulfate and applied directly to a phenyl sepharose column equilibrated in a high ionic stregnth buffer such as 50 mM Hepes, pH 7.5, containing 1.0 mM EDTA and 1.8 M ammonium sulfate. After washing the hydrophobic resin with high ionic strength buffer, a protein fraction containing the ester hydrolase may be eluted with a low ionic strength buffer such as 50 mM Hepes, pH 7.5, containing 1.0 mM EDTA. The eluant may be adjusted to 30% polyethylene glycol to precipitate the proteins and remove residual salts or dialyzed prior to chromography on Blue B Matrex^TM dye resin.

[0019] Ester hydrolase purified from crude commercial lipase preparations or from the fermentation broth of Candida lipolytica strain NRRl-Y1094 migrates as a diffuse band between 60,000 and 120,000 Daltons on a Comassie blue-strained 8% SDS-polyacryalamide gel. Ester hydrolase that has been purified according to the general procedure described above can be treated with either N-Glycanse^R enzyme (Genzyme Corp., Cambridge, MA) or endo-β-N-acetylglucosaminidase H (Endo H) enzyme to remove N-linked carbohydrates when analyzed by SDS-polyacrylamide gel electrophoresis. The treated protein migrates as a single sharp band with a mobility corresponding to a molecule weight of -55,000 Daltons. These data suggest the ester hydrolase to be a glycoprotein. N-terminal sequence analysis of ester hydrolase purified from commercial lipase preparations or from the fermentation broth of Candida lipolytica strain NRRL-Y1094 revealed the following sequence:


This N-terminal sequence defines a novel enzyme as more particularly described herein.

[0020] It has also been found that the crude or purified fractions of the enzyme can be immobilized on various solid supports without loss of stereospecificity or change in stereoselectivity. The solid supports can either be inert adsorbents to which the enzyme is not covalently bonded, but instead is adsorbed by any number of phenomena including, but not limited to, interactions like regions of the inert adsorbent, by hydrogen bonding, by salt bridge formation, or by electrostatic interactions. Inert adsorbent materials include, but are not limited to, synthetic polymers (e.g. polystyrene, poly(vinylalcohol), polyethylene and polyamides), mineralaceous compounds (e.g. diatomaceous earth and Fuller's earth), or naturally occurring polymers (e.g. cellulose). Specific examples of such materials include Celite 545 diatomaceous earth, Amberlite XAD-8 polymeric resin beads and polyethylene glycol 8000. The enzyme may also be immobilized on supports to which the enzyme is covalently bonded (e.g. oxirane-acrylic beads and glutaraldehyde activated supports). Specific examples include Eupergit C oxirane-acrylic beads and glutaraldehyde activated Celite 545. Other possible immobilizing systems are well known and are readily available to those skilled in the art of enzyme immobilization. These immobilized enzyme preparations offer more predictable results, simplify reaction processes and product isolation, and reduce the cost of the enzyme.

[0021] The initial pH of the reaction mixture can be maintained by constant addition of an inorganic base such as sodium hydroxide, or by the use of a suitable buffer. The extent of hydrolysis is monitored by the amount of base added, or by periodic withdrawal of aliquots of the reaction mixture and measuring the relative amounts of starting material and product by high pressure liquid chromatography. For the purpose of making the optically active acid, the reaction may be terminated after 5% to 50% hydrolysis has occurred, but to maximize chemical yield, 40% to 50% hydrolysis is preferred.

[0022] The product is separated from the unreacted ester by adjusting the pH of the reaction mixture to 7.5 - 8, and extracting the ester with an organic solvent such as methylene chloride, ethyl acetate, diethyl ether, or other volatile solvent in which the substrate is stable and soluble, and which is also immiscible in the aqueous phase. Concentration of the organic extracts affords the unreacted ester, while concentration of the aqueous phase yields the acid which can be freed of buffer salts by selective precipitation or chromatography, or other methods known to those skilled in the art.

[0023] Alternatively, the reaction mixture may be acidified, e.g. to pH 3, and both the ester and acid extracted into organic solvents such as methylene chloride, ethyl acetate, diethyl ether, or any volatile solvent in which the substrate is stable, soluble and which is immiscible with the aqueous phase. Concentration of the organic extract yields a mixture of the ester and acid, and these may be separated by selective precipitation or chromatography, or by other methods known to those skilled in the art. Methods for determining the enantiomeric excess of the esters and the acids depend on the nature of the substituent X, and are illustrated by the examples that are described hereinafter.

[0024] The chiral esters are prepared by a method similar to that described for the preparation of the chiral acids. The difference is the hydrolysis is allowed to proceed to 50% to 95%, but in the interest of maximizing the chemical yield, 50% to 60% hydrolysis is preferred. Product separation and isolation is the same as described previously for the chiral acids.

[0025] The esters and acids may each also be prepared as described above using the purified enzyme instead of the crude enzyme. This purified enzyme offers more consistent results and easier isolation of products.

[0026] For the resolution of esters wherein X is NHNHY, and Y is H or COCH₂Ph, a solution or suspension of degassed water or sodium phosphate buffer containing from about 0.001 moles to about 2.0 moles of substrate per litre of reaction volume is adjusted to pH 7.5. A quantity of crude Candida lipolytica enzyme corresponding to 20 mg of protein per gram of substrate is added to initiate the reaction. The reaction mixture is placed under a nitrogen atmosphere and stirred at 36°C. The pH is maintained at 7.3-7.8 by continuous addition of sodium hydroxide until 50% hydrolysis is achieved. The unreacted ester is removed by extraction with methylene chloride, then concentrated in vacuo to afford the (R)-ester in greater than 98% enantiomeric excess. The aqueous phase is concentrated in vacuo to one tenth its original volume, diluted with three volumes of ethanol, adjusted to pH 6.5 with diethylamine, then aged at 0°C to 5°C for several hours to precipitate the (S)-acid. To measure the enantiomeric excess and confirm the absolute stereochemistry of the (R)-esters and (S)-acids, the compounds are converted, by acid hydrolysis, to the (R)-and (S)-3-(3,4-dihydroxyphenyl)-2-hydrazino-2-methylpropanoic acids, respectively. The hydrazino acids are then converted to their respective β-N-(1S)-camphanoyl derivatives and compared to similarly derivatized authentic reference standards by HPLC.

[0027] For the resolution of compounds wherein X represents NH₂, the methods described above can also be advantageously applied. The amino acids prepared in this manner are derivatized with 2,3,4,6-tetra-0-acetyl-B-D-glucopyranosyl isothiocyanate instead of (1S)-(-)-camphanoyl chloride, and the diastereomeric excess of the resulting thiourea is determined by HPLC.

[0028] The following examples further illustrate the various embodiments of this invention.

Example 1

Preparation of (±)-butyl 2-hydrazino-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionate.

[0029] To 100 mL of n-butanol saturated with hydrogen chloride gas at 0°C to 5°C is added 1.98g (8.2 mmol) of 2-hydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionic acid, and the resulting suspension is heated at reflux for 2h. The reaction mixture is then concentrated to dryness in vacuo. The residue is dissolved in sufficient saturated sodium bicarbonate solution to give a solution of pH 7.5-8. The product is extracted with chloroform, dried over magnesium sulfate and then concentrated in vacuo to afford 2.1g (6.95 mmol, 84% yield) of the hydrazino acid ester. Recrystallization from a mixture of methylene chloride and hexane affords 1.96g (80% yield) of the product ester: mp 84.5°C to 87.0°C; HPLC analysis (C-18 column, 280 mm, t_R=16.2 min) showed a single peak; IR(CHCl₃ solution) 3540, 2860, 1720, 1515 cm⁻¹; ¹H NMR (400 MHz, CDC1₃) δ 6.79 (d, 7.6Hz, 1H), 6.62 (s, 1H), 6.59 (d, 7.6Hz, 1H), 4 .08 (m, 2H), 3.82 (s, 3H), 2.99 (d, 14Hz, 1H), 2.74 (d, 14Hz, 1H), 1.58 (m, 2H), 1.34 (m and s, 5H), 0.90 (t, 7.2Hz, 3H).

Example 2

Preparation of (±)-ethyl 2-hydrazino-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionate.

[0030] The procedure of Example 1 is repeated with ethanol instead of n-butanol. The ethyl 2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate is obtained in 79% yield after recrystallization from acetonitrile: mp 114°C to 116°C; HPLC analysis (C-18 column, 280 nm, t_R=5.3 min) showed a single peak; IR (CHC1₃ solutIon) 3540, 2990, 2940, 1710, 1510 cm⁻¹; ¹H-NMR (400 MHz, CDC1₃) δ 6.79 (d, 7.6Hz, 1H), 6.63 (s, 1H), 6.60 (d, 7.6Hz, 1H), 4.14 (q, 7.1Hz, 2H), 3.83 (s, 3H), 2.99 (d, 13.5Hz, 1H), 2.74 (d, 13.5Hz, 1H), 1.34 (s, 3H), 1.24 (t, 7.1Hz, 3H).

Example 3

Preparation of (±)-butyl 2-amino-2-methyl-3-(3,4-dihydroxyphenyl)propionate.

[0031] The procedure of Example 1 is repeated employing 2-amino-2-methyl-3-(3,4-dihydroxyphenyl)propionic acid rather than 2-hydrazino-2-methyl-3-(4-hydrooxy-3-methoxyphenyl)propionic acid. The (±)-butyl 2-amino-2-methyl-3-(3,4-dihydroxyphenyl)propionate is obtained in a yield of 82%. The product is recrystallized from acetonitrile (77% yield) to give pure ester: mp 121.5°C to 122.°C; HPLC analysis (C-18 column, 280 nm, t_R=14.5 min) showed >99% purity.

Example 4

Preparation of (±)-ethyl 2-(β-N-phenylacetylhydrazino)-2-methyl-3-(4-hydroxy-3-methoxyphenyl))propionate.

[0032] To a solution of ethyl 2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate (1.00g, 3.73 mmol) in 9.2 ml of tetrahydrofuran and 9.2 ml of 1,4-dioxane at 0°C to 5°C is added triethylamine (377.4 mg, 3.73 mmol) followed by dropwise addition of phenylacetyl chloride (576.7 mg, 3.73 mmol). The resulting suspension is stirred at room temperature for 2h and then concentrated to dryness in vacuo. Purification of this concentrated product by silica gel chromatography (55% hexane in ethyl acetate) yields 1.01 g (2.62 mmol, 70%) of the (±)-ethyl 2-(β-N-phenyl-acetylhydrazino)-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate: HPLC analysis (C-18 column, 280 nm, t_R=17.6 min); IR (CCl₄ solution) 3050, 3300 (br), 2980, 1720, 1670, 1510 cm⁻¹.

Example 5

Preparation of (R)-ethyl 2-(β-N-phenylacetylhydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate and (S)-2-(β-N-phenylacetyl hydrazino)-2-methyl-3-(4-hydroxy-3-methoxy)propionic acid.

[0033] A solution of (±)-ethyl 2-(3-phenylacetyl)hydrazino-2 -methyl-3-(4-hydroxy-3-methoxyphenyl)propionate (128.0 mg, 0.331 mmol) in 3.40 ml of ethanol is suspended in 43.0 ml of 50 mM sodium phosphate buffer (pH 7.5). Enzyme from Candida lipolytica (26 mg of protein) is added, and the resulting suspension is agitated at room temperature for 41 h, at which time 50% hydrolysis is observed. The reaction mixture is extracted with 3x50 ml of methylene chloride. The combined extracts are washed with water, dried over magnesium sulfate, then concentrated in vacuo to yield 55.3 mg (0.143 mmol) of (R)-ethyl 2-(β-N-phenylacetylhydrazino)-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionate (43.2%). The chemical purity by HPLC assay is >97%. (C-18 column, 280 nm, t_R=17.6 min), with no hydrazino acid present. To determine the enantiomeric purity of the unreacted ester, 5.4 mg of the (R)-ester is heated in a sealed tube with 300 µL of concentrated HCl at 120°C for 2 h to give (R)-2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl)propionic acid. The resulting acid solution is concentrated to dryness in vacuo. The residue is then derivatized with (1S)-(-)-camphanic chloride using a modification of the method reported by Trimble and Vederas (L.A. Trimble and J.C. Vederas, J. Am. Chem. Soc., 1986, 108, 6397). HPLC: (C-18 column, 280 nm, (S,S)-diastereomer t_R=9.2 min, (R, S)-diastereomer t_R=11.7 min). The stereomeric excess is found to be >99% and the (R,S)-stereochemistry of the hydrazide derivative is assigned by comparison to an identically derivatized sample of USP Reference Standard of (S)-2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl) propionic acid.

[0034] The recovered aqueous layers containing the (S)-2-(β-N-phenylacetylhydrazino)-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionic acid are adjusted to pH 3 with IN HCI and then extracted with 3 x 50 ml of methylene chloride. The combined extracts are dried over magnesium sulfate, followed by concentration in vacuo to afford the product. Purification by preparative HPLC affords 6.4 mg (17.9 umol) of (S)-2-(β-N-phenylacetyl hydrazino)-2-methyl-3-(3-methoxy-4-hydroxyphenyl)propionic acid (11% yield). The (S)-2-(β-N-phenylacetylhydrazino)-2-methyl-3-(4-hydroxy-3-hydroxyphenyl)propionic acid is hydrolysed to (S)-2-hydrazino-2-methyl-3-(3,4-dihydroxyphenyl) propionic acid and, then derivatized with (IS)-(-)-camphanic chloride as described above. The derivative is analysed by HPLC (C-18 column) and found to have (S,S) stereochemistry with the diastereomeric excess >99%.

Example 6

Preparation of (S)-2-hydrazino-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionic acid.

[0035] To a solution of 1.2 g (4.1 mmol) of (±)-butyl 2-hydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl )propionate in 1L of sodium phosphate buffer (50 mM, pH 7.5) is added crude Candida lipolytica enzyme (12 mg of protein), and the resulting mixture is incubated in a heated orbit shaker (200 rpm) at 36°C. The reaction is terminated at 20.5% conversion after 17h. The unreacted, optically enriched (R)-butyl 2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate is removed via extraction with methylene chloride. Lyophilization of the remaining aqueous layer and purification of the resulting solid by preparative reverse phase HPLC [6-ords the (S)-2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionic acid in 7% yield. A portion of this product (5 mg) is dissolved in 5 mL of sodium phosphate buffer (0.5M, pH 7.5) and to the resulting solution is added (IS)-(-)-camphanic chloride (50mg in 1 mL of THF) to give the diastereomeric camphanic hydrazide. HPLC analysis of the hydrazide shows the (S,S)-diastereomer to be in >99% diastereomeric excess (C-18 column, 280 nm, (S,S)-diastereomer t_R=21.6 min, (R,R)-diastereomer t_R=26.4 min).

Example 7

Preparation of (S)-2-hydrazino-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionic acid and (R)-butyl 2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl) propionate using purified Candida lipolytica enzyme.

[0036] The procedure of Example 6 is repeated with 10.5 mg of the racemic butyl ester and 9.4 mg of purified Candida lipolytica enzyme in 2 mL of water. The resulting suspension is incubated at 36°C in an orbit shaker (200 rpm). After 4.5 h, 48% of the ester is hydrolysed, and at 16 h, 50% hydrolysis of the ester is observed. The unreacted ester is extracted with methylene chloride and purified by preparative HPLC. The remaining aqueous layer is lyophilized and the residue purified by preparative HPLC. Analysis of the enantiomeric excess of the (R)-ester and (S)-acid via their diastereomeric (1S)-camphanic hydrazide derivatives, prepared as described in Example 5, show both are greater than 99%.

Example 8

Preparation of (R)-ethyl 2-hydrazino-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionate and (S)-2-hydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl) propanoic acid.

[0037] The procedure of Example 6 is repeated employing (±)-ethyl 2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionate rather than (±)-butyl 2-hydrazino -2-methyl-3-(4-hydroxy-3-methoxyphenyl) propionate. The enzymatic hydrolysis is allowed to proceed to 50% conversion and the unreacted ethyl ester is extracted with methylene chloride. A portion of the ester is hydrolysed in concentrated HCl, and then derivatized with (1S)-(-) camphanic chloride as described in Example 5. The camphanic hydrazide derivative was found to be the (R,S)-diastereomer, and the diastereomeric excess was >98%.

Example 9

Resolution of (±)-ethyl 2-hydroxy-2-methyl-3-(4-hydroxy -3-methoxyphenyl)propionic acid.

[0038] To a suspension of 109 mg (0.43 mmol) of (±)-ethyl 2-hydroxy-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionic acid in 4.6 mL of sodium phosphate buffer (50 mM, pH 7.5) is added crude enzyme from Candida lipolytica (7 mg of protein), and the reaction mixture is stirred at 20°C. After 2.5 h, 50% conversion is observed by HPLC (C-18 column, 280 nm, acid t_R=7 min, ester t_R=10.2 min). The reaction mixture is diluted with 20 mL of H₂O and the unreacted ester is extracted with ethyl acetate (2 x 25mL). The combined extracts are washed with saturated sodium bicarbonate solution, dried (MgSO₄) and concentrated in vacuo to afford 27 mg of unreacted ester (HPLC shows one peak). The enantiomeric excess of the unreacted ester was determined by HPLC analysis (Chiracel OD column from J.T. Baker, 280 nm, first enantiomer t_R=7 min, second enantiomer t_R=10.3 min) to be >99% for the faster eluiing enantiomer.

EXAMPLE 10

Spectrophotometric Assay of Ester Hydrolase Activity

[0039] The target ester hydrolase activity was evaluated spectrophotometrically in both crude and purified enzyme preparations using thiopropyl D,L-N-acetyl-α-methylphenylalanine as substrate. The rate of thioester hydrolysis was monitored by coupling the release of propanethiol to a non-enzymic thio-disulfide interchange reaction with dithionitrobenzoate. The production of the resulting thionitrobenzoate anion was monitored at 410 nm. The standard assay mixture contained 50 mM Hepes buffer pH 7.5, 1.0 mM EDTA, 2.0 mM D,L-N-acetyl-α-methylphenylalanine, 1.0 mM DTNB and 10% DMF. One unit of activity is defined as the amount of enzyme required to release 1 µmol of propanethiol per minute at 25°C.

EXAMPLE 11

Isolation of a Crude Protein Fraction Containing Ester Hydrolase Activity From Candida lipolytica Fermetation Broth

[0040] Fermentations of Candida lipolytica strain NRRL Y-1094 (United States Department of Agriculture, Peoria, IL) were carried out at pH 6.0-7.0 at 30°C in medium containing the following constituents: 0.3% yeast extract, 0.3% malt extract, 0.5% Bacto peptone, 0.14% potassium dihydrogen Phosphate, 0.035% disodium hydrogen phosphate and 10% glucose. Enzyme activity was apparent once the culture attained stationary phase with an OD₅₉₀ of approximately 60. A yield of approximately 0.6 mg of esterase per liter of culture was obtained under these conditions with maximum expression typically found after 4-5 days of fermentation. Upon termination of fermentation, the cells were removed by centrifugation at 4000 x g for 30 min and the fermentation broth was adjusted to 1.8 M ammonium sulfate. The cell supernatant from a 14 L fermentation was then pumped onto a 0.5 L phenyl sepharose column that had been equilibrated in 50 mM Hepes buffer, pH 7.5, containing 1.0 mM EDTA and 1.8 M ammonium sulfate. After the culture broth had been applied to the resin, the column was washed with 2 volumnes of equilibration buffer and the protein fraction containing the target ester hydrolase eluted with 1.2 L of 50 mM Hepes buffer, pH 7.5 containing 1.0 mM EDTA. The enzyme fraction was then further concentrated and desalted by adjusted the solution to 30% polyethylene glycol (Av. MW = 8000). The precipitated proteins were then isolated by centrifugation at 7000 x g for 30 min and the resulting pellet was dissolved in approximately 70 mL of 10 mM phosphate buffer, pH 6.5, containing 1.0 mM EDTA. This concentrated, partially purified ester hydrolase fraction may be subjected to column chromatography as described in Example 12.

EXAMPLE 12

Purification of Ester Hydrolase from Crude Candida lipolytica Lipase Preparations

[0041] The purification buffer utilized for all steps was 10 mM sodium phosphate, pH 6.5, containing 1.0 mM EDTA. 10 g crude Candidate lipolytica lipase (Fluka) was suspended in 100 mL buffer and mixed gently for approximately 20 min. Insoluble components were removed from the protein solution by centrifugation at 1,600 x g for 5 min. Solid polyethylene glycol was then added to the clarified supernatant to a final concentration of 30% (w/v) and the solution allowed to mix gently for 30 min at 22°C. The precipitated proteins were then collected by centrifugation at 18,000 x g for 30 min and the resulting protein pellet dissolved in 20 mL buffer. The resulting viscous solution was dialyzed against purification buffer to remove residual polyethylene glycol.

[0042] The crude dialyzed protein fraction was applied to a 100 mL Blue B Matrex™ dye column (Amicon, Danvers, MA) that had been equilibrated in purification buffer. The protein solution was allowed to adsorb to the resin for approximately 30 min and the resin was then washed with approximately 400 mL purification buffer until the absorbance at 280 nm was less than 0.05 absorbance units. The desired ester hydrolase was eluted from the column with approximately 150 mL purification buffer that had been adjusted to 1.2 M KCl. Fractions of 5 mL were collected during the elution step and only those fractions containing enzyme activity were pooled. Typically, the enzyme eluted over 6-7 fractions (-30 mL).

[0043] Ester hydrolase purified using this protocol had a specific activity of approximately 0.07 units mg⁻¹ using the activity assay described in example 10 and the Bradford assay (Bio-Rad, Bethesda, MD) for protein evaluation. The activity of the ester hydrolase was verified by the preparation of (S)-2-hydrazino-2-methyl-3-(4-hydroxy-3-methoxyphenyl)propionic acid using a small scale version of the method described in example 6.

Claims

1. A method for separating by the preferential enzymatic hydrolysis of one enantiomer the enantiomers of racemic carboxylic acid esters corresponding to the following general formula:

wherein
R₁ and R₂ independently represent optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl or optionally substituted aralkyl, or, taken together, complete a cyclic structure incorporating the asymmetric α-carbon atom;
R₃ represents C₁-C₈ alkyl, substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted aryl or optionally substituted aralkyl;
X represents optionally substituted alkyl, halo, amino, alkylamino, aralkylamino, acylamino, hydrazino, alkyhydrazino, aralkylhydrazino, aryldydrazino, acylhydrazino, hydroxy, alkoxy, aralkoxy, aryloxy, mercapto, alkylthio, aralkylthio or arylthio;
and Y represents oxygen or sulfur;
provided that R₁, R₂ and X are different; characterised in that it comprises contacting the racemic ester with an aqueous medium in the presence of an enzyme obtained from Candida lypolytica so as to hydrolyze from 5 to 95% of the ester and separating the hydrolyzed product from the unhydrolyzed starting material.

2. A method as claimed in claim 1 wherein only one enantiomer, preferably the (S)-enantiomer, of the racemic ester is hydrolyzed.

3. A method as claimed in claim 1 or claim 2 wherein the aqueous medium contains a buffer and/or contains up to 75% of an organic solvent.

4. A method as claimed in claim 3 wherein the organic solvent is selected from methanol, ethanol, 1-propanol, 2-propanol, acetonitrile, acetone, 2-butanone, dimethylformamide and dimethylsulfoxide, or methylene chloride, chloroform, tetrahydrofuran, t-butyl methyl ether and ethyl acetate.

5. A method as claimed in any of claims 1 to 4 wherein the hydrolysis is carried out at a temperature of from 0 to 55°C.

6. A method as claimed in any of claims 1 to 5 wherein X represents hydroxy, amino, acylamino, hydrazino or acylhydrazino; or wherein X represents OH, NHR₅ or NHNHR₅; R₅ represents H, COCH₃ or COCH₂pH; R₃ represnts C₁-C₈ n-alkyl; R₁ represents CH₃; and R₂ represents the following:

preferably wherein R₅ represents H; and/or wherein R₃ represents butyl or ethyl; and/ or where in X represents NHR₅; or wherein X represents OR₆; R₆ represents H or OCH₂Ph; R₃ represents C₁-C₈ n-alkyl; R₁ represents CH₃; and R₂ represents allyl.

7. A method as claimed in any of claims 1 to 6 wherein the hydrolyzed enantiomer is produced in at least 95% enantiomeric excess.

8. A method as claimed in any of claims 1 to 7 wherein the enzyme is purified and has the following N-terminal sequence:

9. A purified enzyme characterised in that it comprises the following N-terminal sequence:

10. An isolated nucleic acid characterised in that it comprises a nucleotide sequence encoding a purified enzyme as claimed in claim 9.